In the prior art, AA6000 series aluminum alloys are in increasing demand for structural applications given their desirable mechanical properties of high strength, corrosion resistance and extrudability. These heat treatable aluminum alloys have a wide variety of potential applications including automotive components such as vehicular panels and structural frame members. Examples of these types of aluminum alloys, particularly useful as extrusions, include AA6061, AA6063 and AA6013. Prior to the development of AA6013, AA6061 exhibited the highest strength levels for extrusion purposes.
The compositional limits of AA6013 are identified in U.S. Pat. No. 4,589,932 to Park. AA6013, differing from AA6061 primarily through increased levels of copper and manganese, is reported to exhibit higher strengths than AA6061.
FIG. 1 shows the typical processing steps disclosed in the Park patent for extruding AA6013. The alloy is cast, homogenized and preheated prior to the extrusion step. In order to achieve the improved mechanical properties, the homogenization treatment is practiced at temperatures near the solidus temperature. A minimum of 1,010.degree. F. is identified.
The homogenized and pre-heated billet is then extruded and rapidly cooled followed by a conventional aging treatment to obtain the final extruded product.
However, with the ever increasing demand for improved properties for 6000-series-type aluminum alloys, in particular, extrusions for automotive structural applications, a need has developed to provide methods and alloy compositions which provide improved mechanical properties such as a combination of high strength, high fracture toughness and corrosion resistance. Moreover, improvements are required in processing techniques to achieve increased savings in energy and reductions in operating costs.
In response to this need, the present invention provides both a novel aluminum alloy composition of the 6000-series-type for extrusion as well as improvements in processing techniques for extruding the inventive alloy composition.